Communication systems for communicating between remote locations via a power distribution line are generally well known in the art. Electric utilities typically employ such systems to provide bi-directional communication between an electric generating station and remote customer sites such as homes or office buildings. Such power distribution line communication systems typically operate by modulating a single carrier signal with an outgoing data signal and impressing the modulated carrier signal onto the distribution line for transmission to the remote locations. The modulated carrier signal is then demodulated at the remote locations along the distribution line to recover the outgoing data signal. Thus, for example, an electric utility employing such a system could receive meter data from customer sites without having to send a service person.
Problems with signal reception at certain remote locations arise, however, because the power distribution line typically is open-circuited at one end, or because of other conditions that cause impedance mismatches. Consequently, a single modulated carrier signal propagating along the distribution line is reflected at the open-circuit end due to the large impedance mismatch provided by the open-circuit. The reflection propagates in the opposite direction at the same frequency and combines with the transmitted signal, creating a standing wave along the distribution line. As a result, the modulated carrier signal amplitude is cancelled at fixed locations along the distribution line, severely inhibiting signal reception at these fixed locations. Consequently, for example, electric utilities employing such systems are unable to communicate with customers at those fixed locations.
The distance between fixed locations of signal cancellation is a function of the frequency of the carrier signal, and therefore, carrier signals of different frequencies will experience signal cancellation at different locations along the same distribution line. However, because of the typically large number of remote locations in a power line communication system, some remote locations will experience signal cancellation no matter what frequency carrier is used. Thus, the problem of signal cancellation at certain remote locations cannot be solved simply by using a different carrier frequency.
One possible solution to the problem is to provide signal repeaters to boost the modulated carrier signal amplitude near these fixed locations. Such a solution is described in U.S. Pat. No. 4,357,598 in conjunction with a three-phase power distribution network communication system. While signal repeaters seemingly overcome the problem of signal cancellation at the fixed locations, the addition of signal repeaters increases the cost and complexity of the communication system. Furthermore, installation requires a burdensome determination of the optimum locations for the signal repeaters.
Another possible solution to the problem is to apply a technique referred to as frequency hopping. Such a technique is disclosed in conjunction with a distribution line communication system in U.S. Pat. No. 4,800,363. The frequency hopping technique involves continuously varying the frequency of the single carrier in a single carrier system. Because the fixed locations of signal cancellation are a function of the carrier signal frequency, the locations of signal cancellation will continuously change such that each remote location is reached equally well on average. However, at any given time, certain locations will still experience signal cancellation. Furthermore, the circuitry required to implement the frequency hopping technique is complex and synchronization between transmitters and receivers is required.
Consequently there exists a need for a distribution line communication system and/or method for reducing the effects of signal cancellation due to standing waves, thereby ensuring continuous reception of data at every remote location. Accordingly, the present invention is directed to a power distribution line communication system for reducing effects of signal cancellation at locations along the distribution line due to standing waves caused by reflections of a carrier signal impressed on the distribution line. The present invention satisfies the aforementioned need and overcomes the limitations inherent in the prior art by modulating the same outgoing data signal on a plurality of carrier signals having different frequencies and simultaneously impressing the modulated carrier signals on the distribution line for transmission to remote locations. The carrier signal frequencies are selected such that all of the carrier signals will not cancel at a same location on the distribution line. Thus at least one carrier signal will have a substantial amplitude at each remote location, thereby avoiding the need for signal repeaters and ensuring continuous reception of data at every remote location.